"bluing sand"? - I think possibly that is a 'typo'.

Maybe Mark's post should read: "bluing and ...."

"Parashooter", thanks for your insight on (SA) Krag original finish and arsenal re-finish.

King Carp, maybe your pictured Krag receiver has its original casehardened finish? That light colored area at the front of the magazine floor looks like case-color to me.

FWIW - Attached photos of 'New Old Stock' model 1898 Krag bolt. The recessed bolt face and groove to clear ejector show "black" from heat-treat. The remainder of bolt is 'in the white' from SA polishing.

Grandpa's Gun Parts has NOS bolts, as well as, 'parkerized' bolts, that must have been a WW-1 period development.

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